Here's the original easy-to-build design for a simple open-bolt homemade 9mm submachine gun. The interesting thing is that it could be made by an average person with nothing more than a hacksaw and drill - no machine tools or special skills required.You need a username and password to download from this site, so use this one:

Here's some more gun designs from the same site. They are all good reads, although of course assembling most of the guns would be illegal according to the federal National Firearms Act. Make sure you download the .pdf, I have a hard time reading them otherwise. (you need the free Adobe Reader to view these files)